(1) Technical Field
The present invention relates to a method of producing a semiconductor substrate, and particularly to a method of selectively forming a dielectric layer buried inside substrates which have directly been bonded together with a direct bonding technique.
(2) Prior Art
A dielectric separation method for isolating elements from one another in a semiconductor integrated circuit has recently been attracting attention. There have been proposed various techniques to form a dielectric layer buried inside a substrate to separate element regions from a substrate region.
One of the proposed techniques is disclosed in Japanese Unexamined Patent Publication No. 61-42154. This technique forms a groove on a mirror-polished surface of a first semiconductor substrate. The groove is open to a peripheral end face of the substrate. At the same time, a shallow recess is formed on the first semiconductor substrate or on a mirror-polished surface of a second semiconductor substrate to be bonded to the first semiconductor substrate. The recess communicates with the groove once the first and second semiconductor substrates are bonded together. The polished surfaces of the first and second substrates are attached to each other and bonded together. The bonded substrates are exposed to an oxidizing gas to form an oxide film over the recess.
The inventors of the present invention actually produced dielectric separation substrates according to the above technique and found that, as shown in FIG. 1, a connection A between a groove 53 and a recess 52 was closed with an oxide film 56, thereby preventing an oxidizing gas from flowing into the recess 52. As a result, the inside of the recess 52 could not completely be filled with the oxide film 56. It was found after repetitive tests and consideration that, when a groove and a recess were formed on the surface of a substrate according to chemical etching, dry etching, mechanical polishing, etc., corners thus formed received collective stress and were damaged. The damaged corners oxidized quicker than other regions (other faces in the recessed space) during an oxide film burying process carried out after a substrate bonding process. In particular, corners where the groove communicated with the recess markedly showed such a tendency because they were acute and, therefore, received severe damage. In this way, a portion having a special shape such as a corner locally receives stress and, therefore, quickly oxidizes. The present invention pays particular attention to this sort of damaged portion. When a substrate is made of silicon, stress caused by processing or dry etching the substrate destroys covalent bonds of Si-Si and produces many dangling bonds (nonbonded arms) in the substrate. This occurs at corners in particular. At the corners, oxygen atoms easily penetrate the substrate to promote oxidation of the corners faster than at the other parts, thereby causing the above problem.
The"damaged parts" mentioned in the present invention mean such locations where the above-mentioned damage occurs.